Modified electroconductive polymer material and method for preparation thereof

ABSTRACT

Disclosed is a modified electroconductive polymer material, which comprises a metal filled in a space between the chains of an electroconductive polymer. The metal is oxidized (partially formed as a hydroxide) through a chemical reaction between three substances consisting of the metal, a cation radical/dication, and absorbed water. The metal has a work function less than that of the electroconductive polymer. The contact between the metal and the electroconductive polymer is kept in the state of coexistence between three substances consisting of the metal, the cation radical/dication and the absorbed water. This allows an electroconductive polymer material to have enhanced durability against oxidation/reduction, and controlled conductivity.

TECHNICAL FIELD

The present invention relates to a modified electroconductive polymermaterial having high oxidation/reduction resistance and controlledconductivity, and to a method for producing the modifiedelectroconductive polymer material.

BACKGROUND ART

While it is said that an electroconductive (electrically conductive)polymer are generally excellent in stability against repetitiveoxidation/reduction (doping/dedoping), an electroconductive polymeractually developed to a practical level is only polyaniline, but otherelectroconductive polymers known as wide as polyaniline, such aspolypyrrole and polythiophene, have not been put to practical use as anactive electrical element, primarily due to problems about durability.

As for possible applications of an electroconductive polymer material toactive elements, there have been generally known applications to a holeinjection layer in an organic light-emitting element (Patent Publication1), to an overcurrent protective element (Patent Publications 2 and 3)and to a light-emitting element (Patent Publications 4 and 5), all ofwhich are intended to apply an electroconductive polymer to electricalor electronic elements by itself.

There has also been known a mixed film of an electroconductive polymerand a metal oxide, which is prepared by suspending fine particles ofTiO₂ or SiO₂ in a pyrrole solution as electrolyte to allow a dopant tobe doped homogeneously with mobility, and copolymerizing pyrrole toincorporate the fine particles having a particle diameter of 10 to 1000nm into the obtained polymerized film (Patent Publication 6).

Patent Publication 1: Japanese Patent Laid-Open Publication No.05-114487

Patent Publication 2: Japanese Patent Laid-Open Publication No.09-246010

Patent Publication 3: Japanese Patent Laid-Open Publication No.2002-134303

Patent Publication 4: Japanese Patent Laid-Open Publication No.10-204426

Patent Publication 5: Japanese Patent Laid-Open Publication No.2000-026851

Patent Publication 6: Japanese Patent Publication No. 06-074345

DISCLOSURE OF INVENTION

During the course of an oxidizing/reducing (doping/dedoping) process, alarge voltage may be applied to an electroconductive polymer,particularly in a plus direction, to induce a crosslinking reactionbetween polymer chains. When a film comprises relatively long polymerchains, electrons or holes traveling in the film can move along thechains almost without deviating from the right path to cut through thefilm in a short time of period. Thus, the film exhibits highconductivity.

However, if the polymer chains are crosslinked therebetween to formsideways, electrons or holes will travel in various directions to causeincrease in electrical conduction time. Moreover, such a sideway acts asa pitfall (trap) for electrons or holes in terms of energy, and therebythe lifetime of the trapped electron/hole will run out thereat. Thiscauses lowered conductivity of the film, which leads to ageddeterioration (in extreme cases, the polymer will be changed to anelectrically insulating material, or the functions as anelectroconductive polymer will be lost at this moment).

In addition to this problem, the electroconductive polymer has involvedanother problem. Specifically, while electrons/holes can freely travelin the range of one of the chains, they have to jump to another chainbecause the end of one chain is not connected to the end of anotherchain. This is also regarded as one of the factors causing decrease inelectrical conductivity of an electroconductive polymer.

While the technique disclosed in the above Patent Publication 6 isintended to form a mixed film of an electroconductive polymer and ametal oxide, the incorporation of the metal oxide relies on eventuality,and it is impossible to incorporate the metal oxide homogeneously in acontrolled amount. Moreover, while the metal oxide to be incorporated isin the form of fine particles, the particles have an average particlediameter of about 100 nm, and thereby the thickness of theelectroconductive polymer film has to be increased up to 10 μm or moreto allow the particles to be incorporated in a sufficient amount.

The inventors found that an electroconductive polymer material can haveenhanced durability against oxidation/reduction, and controlledconductivity by bringing a metal susceptible to oxidation into contactwith an electroconductive polymer having a cation radical and a dication(polaron and bipolaron, when expressed by physical terms), and keepingthem under the presence of absorbed water.

Specifically, (1) the present invention provides a modifiedelectroconductive polymer material comprising a metal filled in a spacebetween the chains of an electroconductive polymer. The metal isoxidized through a chemical reaction between three substances consistingof the metal, a cation radical/dication, and absorbed water. The metalhas a work function less than that of an electroconductive polymer.

(2) In the modified electroconductive polymer material set forth in theabove (1), the metal may be one selected from the group consisting ofaluminum, titanium, indium, cadmium, manganese, iron, copper, silver,tin, antimony, lead, sodium and calcium.

(3) The present invention also provides a method for producing themodified electroconductive polymer material set forth in the above (1),comprising the steps of; allowing an electroconductive polymer and ametal having a work function less than that of the electroconductivepolymer to be brought into contact with one another; and keeping thecontact between the metal and the electroconductive polymer, under thepresence of absorbed water, so as to create the state of coexistencebetween three substances consisting of the metal, a cationradical/dication and the absorbed water.

(4) In the method set forth in the above (3), the step of allowing theelectroconductive polymer and the metal to be brought into contact withone another includes: forming on a substrate a film made of anelectroconductive polymer; and vapor-depositing on a surface of the filma metal having a work function less than that of the electroconductivepolymer.

In the electroconductive polymer material of the present invention, aspace between polymer chains is filled with an oxidized metal or metaloxide, such as aluminum oxide or indium oxide, so as to prevent theoccurrence of a crosslinking reaction due to repetition ofoxidation/reduction, and the aged deterioration in the electroconductivepolymer material.

During the process of formation of the metal oxide, a water molecule ishydrated in the metal oxide to inevitably form a metal hydroxide inpart. That is, a part of the metal oxide exists as a metal hydroxide.

For example, when a space between the chains of the electroconductivepolymer is filled with a aluminum oxide having an insulationperformance, the aluminum oxide can prevent the occurrence of acrosslinking reaction, but will hinder electrons/holes from jumping fromthe end of one chain to the end of another chain. That is, while ageddeterioration can be prevented, an electrical conductivity of the entirefilm is lowered. In contrast, if the space is filled with an indiumoxide having a high conductivity, the indium oxide can provide enhancedconductivity while preventing aged deterioration.

According to the production method of the present invention, the amountof the metal oxide to be incorporated in the electroconductive polymeris determined by the amount of vapor-deposited metal, and thereby can becontrolled exactly or properly. In addition, the metal oxide to beincorporated has a small dimensional size of about several nm. Thus,even if an electroconductive polymer film having a thickness of about 1μm or sub-micrometer is used, the compounding (hybridization) can beadequately achieved.

The electroconductive polymer material of the present invention havingstable electrical properties (enhanced durability) can exhibit excellentcharacteristics in applications to various elements, in which inorganicsemiconductors and metals have been monopolistically used, such as acapacitor, an electrode material for secondary batteries, an organiccircuit pattern (organic thin-film transistor etc.), an antistatic sheetor an organic thin-film light-emitting element.

(Function)

Generally, when a film-forming material made of an electroconductivepolymer and dissolved in a solution is oxidized on an electrodesubstrate, it is polymerized in the form of a film. In concurrence withthis polymerization, a reaction causing oxidation of the polymer filmitself is induced to create a cation radical and a dication having apositive charge, in the polymer film.

In the above process, a negative ion (referred to as “dopant”), such asClO₄ ⁻, BF₄ ⁻, PF₆ ⁻ or para-toluene sulfonate ion, may be added intothe solution. In this case, the negative ion is incorporated into thepolymer film, and the cation radical and dication are electricallyneutralized by the incorporated negative ion.

FIG. 1 schematically shows a chemical reaction between three substancesconsisting of a metal, a cation radical/dication, and absorbed water. Asshown in FIG. 1, a metal 1, such as aluminum or indium, which is a metalhaving a work function less than that of an electroconductive polymer 3constituting a film, is attached on the film through a vapor depositionprocess or the like so as to allow the metal 1 to be brought intocontact with the film 3 (see the upper portion in FIG. 1).

In order to more efficiently induce the chemical reaction between threesubstances consisting of the metal, the cation radical/dication, and theabsorbed water, it is desirable to allow the absorbed water to be incontact with the metal and the surface of the film-shapedelectroconductive polymer 3 in a larger area. Thus, a vapor depositionprocess for forming an inhomogeneous pattern, such as island-shapedpattern, may be used as one of preferable techniques. Further, thefilm-shaped electroconductive polymer 3 may have a structural defect,such as micro-void, micro-scratch or pinhole. In this case, amicro-void, micro-scratch or pinhole 4 can be formed in thevapor-deposited metal 1 as shown in FIG. 1 to increase the contact area.

Then, as enlargedly illustrated in the lower portion of FIG. 1, theelectroconductive polymer 3 having the metal 1 attached thereon is keptin the state of coexistence with the absorbed water 2. Thus, theabsorbed water 2 penetrates into the electroconductive polymer 3 throughthe micro-void, micro-scratch or pinhole 4, so that a chemical reactionis induced between three substances consisting of the metal 1, thecation radical/dication 9 and the absorbed water 2, and the metal 1susceptible to oxidation is oxidized (partially formed as hydroxide)while entering into the electroconductive polymer 3.

Further, through a reducing reaction induced in the electroconductivepolymer 3, the cation radical and dication 9 are vanished, and thedopant 8 is dedoped. The formed metal oxide/hydroxide 7 enters into theelectroconductive polymer 3, and diffusingly moves therein to reside ina nano-space between polymer chains 10.

The above result comes from the phenomenon that a galvanic battery isformed between three substances consisting of the metal, the cationradical/dication and the absorbed water, to induce the electron transferfrom the oxidation-susceptible metal having a less work function to thepolypyrrole film having a greater work function. As the result of theelectron transfer, the oxidation-susceptible metal is oxidized through agalvanic corrosion reaction under the presence of the absorbed water,and changed to an oxide of the metal (Al₂O₃/hydroxide (Al₂O₃.xH₂O whenthe metal is aluminum).

FIG. 2 is a schematic diagram showing an electroconductive polymermaterial modified through such a galvanic corrosion reaction. As shownin FIG. 2, an electroconductive polymer is obtained in which thealuminum oxide/hydroxide 7 formed through the chemical reaction betweenthree substances consisting of the metal, the cation radical/dication,and the absorbed water is included in the space between the polymerchains 10. While a part of the dopant 8 and/or an unreacted part of thecation radical/dication 9 will remain after the dedoping if the amountof vapor-deposited metal is insufficient, the level of the remainingamount can be adjusted by the amount of metal to be vapor-deposited.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a chemical reaction between threesubstances consisting of a metal, a cation radical/dication, andabsorbed water, in a production method for a modified electroconductivepolymer material of the present invention.

FIG. 2 is a schematic diagram showing the structure of a modifiedelectroconductive polymer material of the present invention.

FIG. 3 is a graph showing a test result obtained by stepwise cutting asurface of an absorbed aluminum-containing polypyrrole film in InventiveExample 1, and subjecting the film to an elemental analysis based on anX-ray photoelectron spectroscopy analysis after each cutting.

FIG. 4 is a graph showing an aged deterioration in electricalconductivity of each of the absorbed aluminum-containing polypyrrolefilm in Inventive Example 1 and a polypyrrole film having novapor-deposited metal.

FIG. 5(a) is a cyclic voltammogram of a polypyrrole film having novapor-deposited metal.

FIG. 5(b) is a cyclic voltammogram of the absorbed aluminum-containingpolypyrrole film in Inventive Example 1.

FIG. 6 is a graph showing an aged deterioration in electricalconductivity of an absorbed indium-containing polypyrrole film inInventive Example 2.

FIG. 7 is a cyclic voltammogram of the absorbed indium-containingpolypyrrole film in Inventive Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

A polymer to be used in the present invention is not limited to aspecific type, but may be any suitable type, for example, a chainelectroconductive polymer, such as polypyrrole, polyindole,polycarbazole, polythiophene derivatives (including originalpolythiophene: the same is applied to the following derivatives),polyaniline derivatives, poly-acetylene derivatives, poly-furanderivatives, poly-para-phenylene vinylene derivatives, polyazulenederivatives, poly-para-phenylene derivatives poly-para-phenylenesulphide derivatives, poly-isothianaphthene derivatives or poly thiazyl;or a polyacene-based electroconductive polymer.

A process for forming a film made of the electroconductive polymer isnot limited to a specific process, but may be any suitable conventionalprocess, typically, including an electrolytic polymerization process, achemical polymerization process and a solution spreading process. Avapor deposition process may also be used if associated materials have aheat-resistance.

In order to create the state of coexistence between three substancesconsisting of a metal, a cation radical/dication, and absorbed water, ametal is firstly brought into contact with an electroconductive polymer.This may be achieved by one process comprising forming on a substrate afilm made of an electroconductive polymer, and vapor-depositing on asurface of the film a metal having a work function less than that of theelectroconductive polymer.

The metal having a work function less than that of the electroconductivepolymer, for example, aluminum, titanium, indium, cadmium, manganese,iron, copper, silver, tin, antimony, lead, sodium and calcium, isvapor-deposited on a surface of the electroconductive polymer film, andthe contact therebetween is kept under the presence of absorbed water.In this state, the vapor-deposited metal is oxidized (partially formedas a hydroxide), and simultaneously absorbed in the polymer film. Inparticular, an indium oxide exhibiting a high conductivity equivalent tothat of metal, as is well known, may be used to bridge formation toprovide significantly enhanced conductivity in the electroconductivepolymer. In the above process, even if a metal having a relatively largework function, i.e. gold, platinum, nickel, iridium or palladium, isvapor-deposited, the above phenomenon of metal absorption to the insideof the electroconductive polymer will never occur.

Except under vacuum, a substance is generally covered by absorbed water.Thus, the state of coexistence with the absorbed water can be maintainedonly by keeping the contact between the electroconductive polymer andthe metal having a work function less than that of the electroconductivepolymer, in an ordinary or normal atmosphere (e.g. temperature: 20° C.,relative humidity: 50%), or by allowing the metal and theelectroconductive polymer to get wet even if only slightly.

If a vapor deposition process is used, a vapor-deposited metal can besimply placed in a normal atmosphere together with an electroconductivepolymer to obtain a modified electroconductive polymer material havingpreviously-unachievable excellent characteristics, efficiently withoutany cost problem. The process for depositing the metal is not limited tothe vapor deposition process, but may be any other suitable depositionprocess, such as a sputtering process, a plating process, anelectrodeposition process or an electron beam process.

EXAMPLE Inventive Example 1

Through an electrolytic polymerization process using a dichloromethanesolution containing pyrrole (2 mM) and tetraethylammonium perchlorate(65 mM) dissolved therein, as electrolyte, and a glass substratespin-coated with an indium tin oxide (hereinafter referred to as “ITO”)film, as an operation electrode, a polypyrrole film was formed on theITO film.

The conditions of the electrolytic polymerization were set as follows: apolymerization potential of 1.1 V (expressed by a potential relative toa saturated calomel reference electrode), a polymerization temperatureof 0° C. and a supply electrical quantity of 0.7 C/cm². While theelectrolytic polymerization was performed under a nitrogen atmosphere,the nitrogen atmosphere is not essential to the polymerizationatmosphere.

Through this process, a perchlorate ion (ClO₄ ⁻)-doped polypyrrole filmhaving a thickness of about 400 nm was formed on the ITO film.

Then, through a vacuum vapor deposition process, an aluminum metal filmhaving a thickness of about 20 nm was vapor-deposited on a surface ofthe polypyrrole film. The vapor deposition was performed in a vacuumdegree of 10⁻³ Pa at a room temperature (22° C.).

The glass substrate having the aluminum film vapor-deposited on asurface of the polypyrrole film was taken out of a vapor depositionapparatus. Then, an aged deterioration was observed under the conditionthat the glass substrate was left in an air-conditioned room set to havea temperature of 20° C. and a humidity of 50%.

After 5 minutes from the vapor deposition, a part of aluminum in thevapor-deposited aluminum film already started disappearing. Then, thealuminum gradually disappeared, and completely vanished after 12 hours.

FIG. 3 shows a test result obtained by stepwise cutting a surface of theabsorbed aluminum-containing polypyrrole film in Inventive Example 1,and subjecting the film to an elemental analysis based on an X-rayphotoelectron spectroscopy analysis (generally abbreviated as “XPS”) asafter each cutting.

The scale of the right or vertical axis represents a depth from the filmsurface, wherein the point 0 nm indicates the film surface, and thepoint 420 nm indicates the boundary between the ITO film and the glasssubstrate. The horizontal axis represents a bonding energy of an elementof interest, specifically, a bonding energy of a 2p electron of analuminum atom in this graph.

A signal is observed at a bonding energy of 74 to 75 eV. This is asignal of aluminum constituting an aluminum oxide (Al₂O₃) or an aluminumhydroxide (Al₂O₃.xH₂O). While aluminum in the metallic state mustgenerate a signal at 71.4 eV, no signal appears at the value. Thisproves that the vapor-deposited aluminum entered into the polypyrrolefilm, and reached a depth of 150 to 180 nm.

FIG. 4 shows a measurement result of an aged deterioration in electricalconductivity of each of the polypyrrole film having the vapor-depositedaluminum and a polypyrrole film having no vapor-deposited metal. In FIG.4, the zero point of the lapsed time for the polypyrrole film having novapor-deposited metal was defined by a time point immediately after theelectrolytic polymerization, and the zero point for the sample havingthe vapor-deposited aluminum film was defined by a time pointimmediately after the vapor deposition. The measurement was initiatedafter 30 minutes from the zero point of the lapsed time.

In the sample having the vapor-deposited aluminum, an aluminum oxidegets into a space between the electroconductive polymer chains, andthereby the sample has an electrical conductivity reduced to about ¼ to⅕ of that of the film having no vapor-deposited aluminum. However, itshould be noted that the value of the lowered conductivity is still anumeric representing a high-conductivity region, and the sample stillremains in an electroconductive polymer.

FIG. 5(a) is a cyclic voltammogram of a polypyrrole film having novapor-deposited metal.

FIG. 5(b) is a cyclic voltammogram of the polypyrrole film having thevapor-deposited aluminum film. When a positive potential is applied tothe polypyrrole film, a current will flow in a plus direction. Thiscorresponds to the state when the film is oxidized to create a radicalcation and a dication, and ClO₄ ⁻ enters from the electrolyte into thefilm to electrically neutralize the cation radical/dication.

As seen in FIG. 5(a), the waveform gradually squashes along withrepetition of potential sweep, and changes to have an oval shape. Thisbehavior is observed when the film has a lowered electricalconductivity, and an increased electrical resistance. That is, FIG. 5(a)shows that the film is gradually deteriorated. In contrast, as seen inFIG. 5(b), the waveforms overlap from the time around the 4th sweep, andthe film has stable oxidation/reduction. This proves that durability ofthe polypyrrole against oxidation/reduction is significantly improved.

Inventive Example 2

Except that a metal to be vapor-deposited was changed from aluminum toindium, a sample having an indium film vapor-deposited on a surface of apolypyrrole film was prepared under the same conditions as those inInventive Example 1.

The phenomenon of disappearance of the indium was observed in the samemanner as that in Inventive Example 1.

On the analogy of the aluminum case, it is proven that the indiumreacted with cations (cation radical/dication) and absorbed water in thepolypyrrole, and incorporated in the film in the form of a modifiedtransparent substance consisting of indium oxide (In₂O₃)/indiumhydroxide (In₂O₃/xH₂O) (slightly yellow-tinged substance)

FIG. 6 shows an aged deterioration in electrical conductivity of thepolypyrrole film having the indium oxide/indium hydroxide absorbedtherein, through a 4-terminal measuring method, wherein the zero pointof the lapsed time is defined by a time point just aftervapor-deposition of the indium. As seen in FIG. 6, while theconductivity of the sample is lowered until a lapsed time of 5 hours, astabilized conductivity has a high value of 2000 S/cm, which is 34 timesof that in the polypyrrole film before the vapor deposition. This can beunderstood as meaning that the indium oxide resides in the polypyrrolefilm, and contributes to increase in conductivity, as described above.

FIG. 7 shows a cyclic voltammogram of the polypyrrole film having theindium oxide/indium hydroxide absorbed therein. As seen in FIG. 7, theincorporation of an indium oxide in a polypyrrole film also has theeffect of providing enhanced durability of the film.

Comparative Example 1

Except that poly (methyl methacrylate), which is a polymer having aninsulating performance, was used in place of pyrrole, an aluminum filmwas vapor-deposited on a surface of a poly (methyl methacrylate) filmunder the same conditions as those in Inventive Example 1, whereintetraethylammonium perchlorate was dispersed in the poly (methylmethacrylate) film. This state means that vapor-deposited aluminum andabsorbed water coexist, but no cation radical/dication exists. In thisexample, no disappearance of the aluminum film occurred. This verifiesthat the phenomenon of disappearance of the aluminum is based on thepresence of a cation radical or dication, and requires inducing the flowof electrons from the aluminum to the polymer film

Comparative Example 2

Except that a metal to be vapor-deposited was changed from aluminum togold, a sample having a gold film vapor-deposited on a surface of apolypyrrole film was prepared under the same conditions as those inInventive Example 1.

In the polypyrrole film having a noble metal, such as goldvapor-deposited thereon, no metal was incorporated into the polypyrrolefilm even after a lapse of a one week from the vapor deposition. Whengold is used, no electron flow from the aluminum to the polymer film isinduced because gold has a work function approximately equal to that ofpolypyrrole. This also demonstrates the importance of the electron flow(current).

Comparative Example 3

The aluminum/polypyrrole sample prepared in Inventive Example 1 wasstored under vacuum (10⁻³ Pa) instead of in the air. As a result, nodisappearance of the aluminum occurred even after a lapse of 24 hours.This reason is that a sufficient amount of absorbed water could notobtained from the vacuum atmosphere, and thereby the state ofcoexistence between three substances consisting of the aluminum, thecation radical/dication, and absorbed water could not be created.

INDUSTRIAL APPLICABILITY

According to the present invention, high durability againstoxidation/reduction and enhanced control of conductivity, which couldnot be obtained in electroconductive polymers prepared throughconventional techniques, can be achieved. Thus, the present inventioncan contribute to the practical use of an electroconductive polymer as amaterial for various electrical and electronic elements.

1. A modified electroconductive polymer material comprising a metalfilled in a space between the chains of an electroconductive polymer,said metal being oxidized through a chemical reaction between threesubstances consisting of said metal, a cation radical/dication, andabsorbed water, said metal having a work function less than that of theelectroconductive polymer.
 2. The modified electroconductive polymermaterial as defined in claim 1, wherein said metal is one selected fromthe group consisting of aluminum, titanium, indium, cadmium, manganese,iron, copper, silver, tin, antimony, lead, sodium and calcium.
 3. Amethod for producing the modified electroconductive polymer material asdefined in claim 1, comprising the steps of; allowing anelectroconductive polymer and a metal having a work function less thanthat of said electroconductive polymer to be brought into contact withone another; and keeping the contact between said metal and saidelectroconductive polymer, under the presence of absorbed water, so asto create the state of coexistence between three substances consistingof said metal, a cation radical/dication and said absorbed water.
 4. Themethod as defined in claim 3, wherein said step of allowing saidelectroconductive polymer said metal to be brought into contact with oneanother includes: forming on a substrate a film made of anelectroconductive polymer; and vapor-depositing on a surface of saidfilm a metal having a work function less than that of saidelectroconductive polymer.